Since the mid-1950s, bronze leaf, a disease caused by the fungus Apioplagiostoma populi, has damaged and killed aspen hybrids in plantations and landscape plantings in the Midwest, Northeast, and Canada. Promising hybrids have been eliminated from future planting because of their susceptibility. The disease is common among naturally occurring hybrid trees of seed origin on disturbed sites such as roadsides and railway and power line right-of-ways. Before Forest Service scientists studied this fungus, its complete life cycle, pathology, and host range were unknown. The fungus cannot be grown on artificial media and produces a toxin involved in the disease syndrome. The scientists identified a second spore stage involved in the fungus life cycle and found evidence suggesting that only aspen hybrids, not pure species, are susceptible. The unique pathogen acts as a genetic barrier to natural hybridization among several native and introduced aspen and poplar species, thus preserving the pure species in nature and acting as an obstacle to breeding in aspen improvement efforts.